Symmetric, or single-key, encryption, is a widely used technique for encryption by which an intelligible message, referred to as plaintext, is converted into apparently random nonsense referred to as ciphertext. Symmetric encryption uses an encryption algorithm and a key, where the key is typically independent of the plaintext. In symmetric encryption, the encryption algorithm must be such that it is impractical to decrypt the message on the basis of the ciphertext by itself. The security of symmetric encryption, thus, depends on the secrecy of the key, not on the secrecy of the encryption algorithm.
Statistical analysis is a typical technique of cryptanalysis used to break encrypted ciphertext. Diffusion and confusion are two basic cryptographic principles used to thwart such cryptanalysis. Using the principle of diffusion, the statistical structure of the plaintext is dissipated into long-range statistics of the ciphertext. Diffusion is typically achieved by having each plaintext digit affect the value of many ciphertext digits. Using the principle of confusion, the relationship between the statistics of the ciphertext and the value of the encryption key is made to be as complex as possible. Confusion is typically achieved using a complex substitution algorithm.
In existing symmetric encryption, encryption of the same plain text at multiple instances of time using the same encryption algorithm and the same key produces the same ciphertext.